Volatile substances, such as essentially hydrocarbons, and to a lesser extent other volatile components evaporate in a fuel tank of a motor vehicle as a function of the pressure and temperature conditions prevailing in the fuel tank as well as a composition of the fuel. For reasons of environmental protection and safety, these substances must be captured and supplied to the engine for combustion. For this purpose, the volatile substances are generally adsorbed and temporarily stored with the aid of an active carbon filter. To regenerate or desorb the active carbon filter, the substances are extracted with the aid of a fluid flow—generally fresh air—and supplied to an intake manifold provided upstream from the internal combustion engine for combustion. The extraction takes place with the aid of underpressure, which sets in the intake manifold as a result of throttling of the engine.
In turbo engines, hybrid vehicles and engines in which the engine may be operated in dethrottling mode to reduce fuel consumption, the problem generally arises that a conventional fuel tank ventilation using underpressure in the intake manifold does not adequately regenerate the active carbon filter.
In addition, the laws of some countries require the operability of fuel tank venting systems in motor vehicles to be tested with the aid of on-board means, i.e., using a so-called on-board diagnosis (OBD). During the on-board diagnosis, any leaks which occur must be detected, signaled and corresponding data provided to an on-board memory for off-board diagnosis to be carried out in a repair shop.
A treatment system for evaporating fuel is discussed in DE 101 54 360 A1 which has a purge channel for providing a connection between an intake manifold of an internal combustion engine and a fuel tank. In an intermediate part of the purge channel, a canister is provided for temporarily adsorbing evaporated fuel which is produced in the fuel tank. A purge pump is furthermore provided in one section of the purge channel, the purge pump being configured to transport evaporated fuel from the canister to the intake manifold. After an atmospheric air inlet valve of the canister closes, the operation of the purge pump is interrupted when the underpressure in the fuel tank has reached a predetermined value as a result of the operation of the purge pump. After the purge pump stops, a flow control valve is closed, which is provided on the intake manifold or in its vicinity. If a change in the pressure in the fuel tank is detected after a predetermined period of time has elapsed, the entire purge channel is checked in reference to an error.
A device for diagnosing a fuel tank venting system of a vehicle is furthermore discussed in DE 197 35 549 A1, which includes a fuel tank and an adsorption filter which is connected to the fuel tank via a fuel tank connecting line. For venting, the fuel tank venting system has a fuel tank venting valve which is connected to the adsorption filter via a valve line. A switching means alternately applies a pressure to the fuel tank venting system and a reference leak with the aid of an on-board pressure source and ascertains a possible leak in this way.